1. Field of the Invention
The present invention relates generally to improvements in the field of optical fiber, and more particularly to advantageous aspects of improved optical fibers with reduced splice loss and methods for making same.
2. Description of Prior Art
In the optical fiber industry, there is an ongoing effort to develop new types of fibers, such as erbium doped fibers (EDFs) and the like. However, it has proven to be difficult to maintain desired performance when these fibers are spliced to other fibers, particularly fibers having different modefield diameters (MFDs) and refractive index profiles (RIPs), such as a standard single mode fiber (SSMF).
It has been found that splicing an EDF to an SSMF results in a splice loss that is typically 0.1-0.2 dB higher than when other types of fiber are spliced to an SSMF. It is important to maintain low splice loss in order to obtain an acceptably low level of noise in such devices as an erbium doped fiber amplifier (EDFA). Maintaining low splice loss also typically results in an improvement in overall power conversion efficiency.
Achieving acceptable performance for EDFs is especially challenging in an optical amplifier, where low splice loss may be required for a wide signal wavelength range. An EDFA, for example, may be pumped at a wavelength of 980 nm to yield gain at 1550 nm. In this case, low splice loss may be required both around 980 nm and around 1550 nm.
In an EDFA in which an EDF is spliced to an SSMF, splice loss resulting from modefield mismatch may be reduced by thermally diffusing the refractive index profile (RIP) of the EDF during splicing, so that the RIP of the EDF approximates the RIP of the SSMF. In this case, the two fibers will have approximately the same evolvement of the MFD with respect to wavelength at the splice point.
However, it has proved difficult to use a thermal diffusion technique to obtain an RIP for the EDF with the desired degree of similarity to the RIP of the SSMF, while maintaining an acceptably low cutoff wavelength. As mentioned above, an EDFA is commonly pumped at a wavelength of 980 nm. In order to ensure single mode propagation of the EDF in the amplifier in this situation, the cutoff wavelength of the EDF must be maintained at a level below 980 nm.
Further, splice loss issues are aggravated by variations in splice conditions, particularly in a volume production environment. These variations are caused by a number of factors that may be difficult to control, including worn splice electrodes and fluctuations in current control, as well as environmental conditions, such as humidity, temperature, pressure and the like. With current fiber designs, these variations can contribute significantly to splice loss.